When you are asked to write a personal statement, whether you are applying for graduate school, off-campus study, or a job, a few basic rules will help you produce a good one!
Remember your audience
You are writing to a committee who needs to know who you are as a person, as distinguished from all the other applicants. Make yourself stand out from the crowd.
Remember your thesis: "You really want me."
Every sentence should constitute evidence of this thesis. If a sentence does not serve as evidence of that claim, eliminate it. Under this advice, for example, you should eliminate sentences that say "such and such a program offers an incomparable experience."
Remember that readers like stories
If your essay merely replays your resume, the reader will gain little from it. Rather than recite a list of experiences, tell a story from your life to illuminate the thesis.
Make every word count
Never write a sentence anyone else could write. Make every sentence distinguish you from all the other applicants. Don't tell your readers things they already know, such as how great their program is. Also, remember that your audience has to read a lot of essays: concision will please them; repetition or wordiness will do the opposite.
Show, don't tell
If you tell the reader that you are intelligent and accomplished, those abstractions are likely to pass through the reader's mind without sticking. If you show that you have those qualities by giving specfic examples and telling illustrative stories, you will garner their attention.
These rules do not make such writing easy, but following them can make your essay memorable!